This article is intended to illustrate how to switch to a shared email account within the Outlook Online Application as part of Office 365.
After signing into Outlook Online, click on you user icon as displayed below
A menu will have opened, named My Accounts. Select "Open another mailbox. . . " as shown below
Type the email address that you intend to access and click open.
If you are using the outlook client, you will notice the Shared mailbox on the left pane of outlook. To send email, you will noticed an option on the outlook client From.
Click to select drop down to send from preferred email account.